1. Field of the Invention
The invention relates to a hydrodynamic type bearing, particularly a hydrodynamic type oil-impregnated sintered bearing, and to a hydrodynamic type bearing unit employing the same. Hydrodynamic type oil-impregnated sintered bearings are especially suited to bearings in use for information equipment, i.e., bearings for disc drives in optical disc devices including DVD-ROMs and DVD-RAMs, magneto-optical disc devices including MOs, and magnetic disc devices including HDDs and high capacity floppy disc drives (FDDS) such as HiFDs and Zips, or bearings for polygon scanner motors in LBPs and the like. In particular, hydrodynamic type oil-impregnated sintered bearings are suitably applied to bearings in thinner models of motors.
2. Description of the Prior Art
Spindle motors for the information equipment mentioned above are required for further improvements in high-speed rotational accuracy, higher speeds, lower costs, lower noise, and the like. One of the component parts determining these performance requirements is a bearing for supporting the spindle of a motor. In recent years, studies have been made on the use of a hydrodynamic type bearing, especially of a so-called hydrodynamic type oil-impregnated sintered bearing, as such bearing. In a hydrodynamic type oil-impregnated sintered bearing, the bearing body of sintered metal is impregnated with lubricating oil or lubricating grease, and a lubricating film is formed in a bearing clearance by means of the hydrodynamic action of hydrodynamic pressure generating grooves provided in the bearing surface, so as to support a spindle without contact. This hydrodynamic type oil-impregnated sintered bearing, having the features of high rotational accuracy, low noise and the like despite of its low costs, appears to well meet the aforesaid performance requirements.
FIG. 12 shows an example of a spindle motor in an optical disc device, employing a hydrodynamic type oil-impregnated sintered bearing 1. As shown in the figure, this spindle motor comprises the hydrodynamic type oil-impregnated sintered bearing 1, a housing 2 for containing the bearing 1, a rotating shaft 3 supported by the bearing 1, a turntable 5 and a damper 6 for supporting and fixing an optical disc 4, and a motor section M composed of a stator 7a and a rotor 7b. The spindle motor is configured so that energizing the stator 7a brings a rotor case 8 integrated with the rotor 7b, the turntable 5, the optical disc 4, and the damper 6 into integral rotation.
The hydrodynamic type oil-impregnated sintered bearing 1 is composed of a porous bearing body formed in a thick cylindrical shape, and oil stored in the pores of the bearing body by means of the impregnation with lubricating oil or lubricating grease. In the inner periphery of the bearing body, a pair of bearing surfaces opposed to the outer periphery of the rotating shaft via bearing clearances are formed so as to be axially separated from each other. In each bearing surface are formed hydrodynamic pressure generating grooves slanting against an axial direction.
As shown in FIGS. 12 and 13, a thrust load on the rotating shaft 3 is supported by a thrust bearing 9 arranged at the bottom of the housing 2. The thrust bearing 9 typically has a configuration (so-called a pivot bearing) in which the spherical shaft end thereof slides on a resin washer 9a having high lubricity provided at the bottom of the housing 2.
The pivot bearing, however, may suffer a change in shaft position with lapse of time due to a recess in the washer 9a created by elastic deformation, plastic deformation, deformation from friction, and the like of the washer 9a. The change in shaft position cause variations in disc position in the cases of HDD devices, and variations in mirror position in the cases of polygon scanner motors in LBPS, greatly affecting the motor performance. As measures against this, the washer 9a could be formed of metal material or ceramic material; in such case, however, the shaft will be worn out to turn the spherical surface of the shaft end into a flat surface, possibly producing the problems of a change in shaft position, a rise in torque, fluctuations in torque, and the like.
Moreover, in recent years, the spindle motors are often required for thinner models in view of the mounting of optical disc devices and HDD devices on notebook type computers and the like, while the configuration that the bearing surfaces 1b are arranged axially at two places as described above has a limit in obtaining thinner models. As shown in FIG. 14 for example, a thinner model can be obtained by arranging the bearing surface 1b at only one place. This produces, however, the problem of a decrease in rigidity with respect to moment loads. In other words, since the rotating shaft 3 at the portion projecting from the bearing 1 is subjected to eccentric loads from the rotor case 8 having the rotor magnet 7b fixed thereto, the disc 4, the turntable 5, the clamper 6, and the like, it is feared that the accuracy in shaft run-out might be deteriorated by the moment loads.
Such hydrodynamic type bearing has hydrodynamic pressure generating grooves of herringbone type, spiral type, or the like for generating a hydrodynamic pressure formed in the inner periphery (radial bearing surface) of its almost-cylindrical-shaped sleeve material. A conventional method for forming hydrodynamic pressure generating grooves is known in which a rod-shaped jig, holding a plurality of balls harder than the bearing material arranged circumferentially at equal intervals, is inserted into the inner periphery of the bearing material, and the jig is rotated and fed to put the balls into spiral movements while pressing the balls against the inner periphery of the material to form by rolling (plastic working) the hydrodynamic pressure generating grooves (Japanese Patent No. 2541208).
In such hydrodynamic type bearing, a thrust bearing surface having hydrodynamic pressure generating grooves is sometimes provided on an end face of the bearing or a surface opposed thereto of the spindle, in order to non-contact support the spindle in a thrust direction. These hydrodynamic pressure generating grooves in the thrust bearing surface are typically formed by pressing.
The above-described rolling of hydrodynamic pressure generating grooves, however, creates heaving at regions adjacent to the hydrodynamic pressure generating grooves in working. The heaving must be removed by a lathe or a reamer (Japanese Patent Laid-Open Publication No.Hei 8-232958), complicating the processes. Besides, in the removing, positioning need to be performed with end faces of the bearing pressed against the jig; therefore, the end faces of the bearing must have been finished with a high degree of accuracy, and the accuracy of the end faces should be maintained in the removing as well, making the work troublesome.
Moreover, since the thrust bearing surface is worked in a separate process from that of the radial bearing surface, a bearing surface formed in the preceding process may decrease in accuracy during the following process, producing a difficulty in quality control.